The invention relates to a process for monitoring the conversion capacity of a particulate filter arranged in an exhaust gas duct of an internal combustion engine to oxidize nitrogen monoxide to nitrogen dioxide over a catalytic coating, by determining a flow resistance of the particulate filter and by determining the temperature of the exhaust gas upstream of or in the particulate filter.
Particulate filters are used to reduce particulate emissions from diesel engines in the exhaust gas duct of internal combustion engines. The exhaust gas is passed through the particulate filter, which separates out the solid particles present in the exhaust gas and retains them in a filter substrate. As a result of the masses of soot stored in the filter substrate, the particulate filter becomes blocked with time, which is manifested in an increase in the flow resistance and hence in the exhaust gas backpressure. For this reason, the mass of soot stored has to be discharged from time to time.
Legislation requires monitoring of all emission-relevant components, such as exhaust gas aftertreatment components and the accompanying sensor system, in the context of “on-board diagnosis” (OBD) for compliance with thresholds, which are usually specified as a multiple of the emission thresholds. One aspect here is the monitoring of the coating of a particulate filter for a sufficient ability to oxidize nitrogen monoxide to nitrogen dioxide and hence to provide a suitable reagent for an SCR catalyst arranged downstream in the exhaust gas duct (SCR=Selective Catalytic Reaction), in which nitrogen oxides are converted by means of urea to carbon dioxide, nitrogen and water. By monitoring the function of the coating, its ability to oxidize hydrocarbons would simultaneously also be monitored. For diesel oxidation catalysts, such monitoring of the catalytic coating could likewise be required.
According to the prior art, in general, monitoring of components in the context of on-board diagnosis requires restriction of the operating parameters under which the monitoring can be carried out. To improve the distinction between a marginal reject component (best part unacceptable) and a marginally acceptable part (worst part acceptable), the plausibilization functions are then carried out only for a restricted range of one or more of the following parameters: exhaust gas mass flow, exhaust gas volume flow, exhaust gas temperature, speed, injection rate, vehicle speed, ambient pressure, ambient temperature or exhaust gas recycle rate. It is also possible to provide restrictions for mode of operation, status, run time or service life of the internal combustion engine, or for regions of the signals for nitrogen oxide, hydrocarbon, carbon monoxide, particulate mass or oxygen concentration. Some monitoring methods also envisage performing the monitoring only in the case of steady-state or quasi-steady-state operating conditions of the internal combustion engine.